Anyone would know how much for cab to Durnstein and which company offer the best price?
Its about 100 km, i guess it will be around € 200, may be a little cheaper if you can speak with one of the smaller companies listed in Herold (but they might not speak english)
If that is too expensive for you, you could take the train until Krems and take a cab from there.
You can first take train S7 from the airport to the center of Vienna and get off at Wien Handelskai. Then change to S45 and get off at Wien Heiligenstadt. From there the trains to Krems are departing. The train journey from the airport to Krems will take you 2 hours and 12 minutes. The cost for the train journey is 20.90 € for one adult.
From Krems it is only 15 km by cab so you could expect to pay around 30 €
There is also a bus from the train Station in Krems to Dürnstein, but it is only departing every hour.
Best regards, Christina